Heat exchanger and method of making

ABSTRACT

A heat exchanger of increased effectiveness is disclosed. A porous metal matrix is disposed in a metal chamber or between walls through which a heat-transfer fluid is directed. The porous metal matrix has internal bonds and is bonded to the chamber in order to remove all thermal contact resistance within the composite structure. Utilization of the invention in a rocket chamber is disclosed as a specific use. Also disclosed is a method of constructing the heat exchanger.

ORIGIN OF THE INVENTION

This invention was made by employees of the U.S. Government and may bemanufactured or used by or for the Government of the United States forgovernmental purposes without any payment of royalties thereon ortherefor.

This is a division of application Ser. No. 856,462, filed Nov. 30, 1977,now U.S. Pat. No. 4,199,937.

BACKGROUND OF THE INVENTION

This invention relates to apparatus for transferring heat from a firstenvironment to a second environment of lower temperature and is directedmore particularly to devices known as heat exchangers.

Basically, a heat exchanger includes a wall of high heat conductivitymetal separating two environments of different temperatures. If the wallis the shell of a device such as a rocket chamber, for example, whichmust be protected from destruction by the combustion inside the rocketchamber, a coolant medium or fluid may be directed or flowed over theouter surface of the wall to absorb the heat.

Other examples of heat exchangers include the automobile radiator whichcomprises a plurality of tubes through which there is directed theliquid coolant from an engine. The temperature of the coolant is reducedby air directed against the radiator tubes.

In the past, attempts to increase the heat transfer have includedroughening the surface of the wall in contact with the flowing fluid,increasing the fluid velocity by decreasing the cross-sectional area ofthe coolant tubes or channels and/or increasing the pressure drop acrosseach channel to increase fluid velocity. Additionally, the use of finson the dividing wall or which extend radially inwardly or outwardly fromthe cooling tubes have been utilized.

Some of the problems which result from previous prior art attempts toincrease the heat transfer include lack of control of surface roughness,low cycle thermal fatigue problems which result from joining differentsections of material by welding or brazing or the like, the difficultyof machining suitable size channels in thin walls and confined spaces,and the difficulty of machining fins in small channels.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a new and novel heatexchanger having high heat-transfer enhancement.

It is another object of the invention to provide a heat exchangerwherein the velocity of a flowing heat-transfer medium is maintained ata relatively low magnitude, thereby avoiding the need for special pumpsto develop a high pressure for the heat-transfer medium.

Still another object of the invention is to provide a heat exchanger anda method of making the exchanger, both of which are relatively simple ascompared to the prior art.

Yet another object of the invention is to provide for a rocket engine, aheat exchanger which will cause maximum heat transfer to a coolant fluidat the rocket throat where inner wall temperatures are the highest.

A still further object of the invention is to provide a method of makinga rocket chamber having maximum heat transfer at the throat area.

In summary, it is an object of the invention to provide a heat exchangerof high effectiveness which is simple in construction and which may beeasily altered to provide areas of increased heat transfer and which isrelatively simple to construct.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial section of the heat exchanger embodying theinvention.

FIG. 2 is a half axial section of the rocket chamber embodying theinvention.

FIG. 3 is a partially cutaway oblique view of a rocket chamber embodyingthe invention.

FIG. 4 is a partial cross section of the rocket chamber of FIG. 3 takenalong the lines 4--4.

FIG. 5 is a view of a modification to the structure shown in FIG. 4.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a heat exchanger 10 whichcomprises a tubular shell 11 connected through a supply conduit 12 andthrough a pump 13 to a heat-transfer medium or fluid supply 14. Whilethe heat exchanger will be discussed as being in a high temperatureenvironment and with the coolant being supplied from the heat-transfermedium supply 14, it will be understood that the heat exchanger can beutilized to transfer heat in the opposite direction, that is theheat-transfer medium would be of higher temperature than the environmentoutside of the heat exchanger. The coolant flows from supply 14 throughthe conduit 12 and pump 13 and the heat exchanger 10, as shown by thearrows 15.

Disposed in the tube 11 is a porous metal matrix 16 which has a greaterdensity as at 17 then it does at the ends of the tube 11. This increaseddensity of the porous metal matrix at 17 increases the internalheat-transfer coefficient of the porous matrix in an area where thetemperature of the environment is much greater than at the end of tube11 where the coolant enters or at the opposite end where the coolant isexhausted. The high temperature and/or heat flux is represented byarrows 18 while the lower temperatures and/or heat flux toward the endsof tube 11 are represented by the short arrows 19.

While the increased density of the porous metal matrix as at 17 willincrease the heat transfer, it will also reduce the volume of coolantflowing through tube 11 if it is of constant diameter. To the end thatthe reduction of coolant flow volume caused by the increased density ofthe porous metal matrix at 17 will be minimized or eliminated, thediameter of tube 11 is increased as at 20, thereby providing a maximumcross-sectional area where the density of the porous metal matrix isgreatest so that the overall flow rate and pressure drop is equivalentto that of a straight tube.

Ideally, the density of the porous metal matrix 16 should increase ordecrease proportionally in accordance with the temperature and/or heatflux being impressed on the tube 11. In other words, the density of theporous metal matrix 16 preferably follows the temperature profile of theheat flux along the length of the tube 11. Likewise, the increaseddiameter of tube 11 at 20 preferably followed the temperature and/orheat flux profile curve and, of course, will be maximum where heat fluxand/or temperature is maximum and minimum where heat flux and/ortemperature is minimum.

The heat exchanger 10 may be constructed by forming a body of porousmetal, as for example, sintered metal powder, sintered fibers, wovensintered wire cloth or foam metal, or other porous metal bodies such asphotoetched plates or laser drilled plates with randomly arranged poresor openings into the desired shape which may be cylindrical and whichmay include one or more of the increased diameter portions, as shown at20 in FIG. 1. A tube 11 may then be formed over the porous metal matrixby electroforming, sputtering or the like.

As an alternative, the heat exchanger 10 may be constructed by firstforming tube 11 including, if desired, one or more portions of increaseddiameter. Such methods are well known in the art. Tube 11 may then befilled with sintered metal powder, metal fibers or the like. The tube 11and its contents are then heated to a temperature high enough to causebonding or welding of the contents of tube 11 to form a porous metalmatrix and to bond or weld to the tube 11. The bonding or welding of allpoints of metal to metal contact provides maximum heat transfer byeliminating normal high thermal resistance contact points.

The metals used for the tube 11 and for the porous metal matrix 16 arethose having the best heat-transfer characteristics. However, theselection of metals will depend upon the environment and other materialswith which the heat exchanger comes in contact, and will also dependupon the coefficients of expansion of the metals used for the porousmatrix 16 and the tube 11. In forming the heat exchanger 10, sputteringmay have some important advantages in that a greater number of alloyscan be deposited over the porous metal matrix than can be deposited byelectroforming.

Referring now to FIG. 2, there is shown a half axial section of a rocketchamber 21 having an inner annular wall 22 and an outer annular wall 23configured so as to produce a throat 24. The space between the innerwall and the outer wall 23 contains a porous metal matrix 25 which isbonded or welded to the inner and outer walls 22 and 23, respectively,by suitable bonding or welding techniques. A coolant fluid is directedto a manifold 28 at the downstream end of the rocket nozzle and flowsthrough the matrix 25 in an upstream direction, i.e., opposite to theflow of combustion gases exiting as shown by arrow 29. Of course, thecoolant flow may be in the opposite direction to that shown in FIG. 2,if desired. The distance between walls 22 and 23 is much greater at thethroat area as indicated by arrows 26 than it is at the upstream ordownstream ends, as indicated by arrows 27. Additionally, the density ofthe porous metal matrix 25 may be much greater at the throat area 24than it is at the upstream or downstream ends of the chamber, as isshown in FIG. 2.

FIG. 3 is a partial cutaway view of a rocket chamber similar to thatshown in FIG. 2 and corresponding parts are identified by like numerals.In the rocket chamber 21 of FIG. 3, elongated porous metal matrix slugs30 are disposed in an annular configuration around the throat 24 betweeninner and outer walls 22 and 23. The slugs 30 are separated bylongitudinal ribs 31 which extend substantially the full length of therocket chamber. Ribs 31 and the slugs 30 are bonded or welded to theinner wall and to the outer wall and, in addition, each rib is bonded orwelded to the slugs 30 adjacent to it. The slugs 30 increase the heattransfer in the area of the throat where inner wall temperature ismaximum to provide maximum protection of the rocket chamber fromdestruction. It will be understood that the slugs 30 shown in FIG. 3 mayextend the full length of rocket chamber 21 or any part thereof, asdesired. Also, in accordance with the construction shown in FIGS. 1 and2, the density of the porous metal matrix may be selectively varied toprovide additional heat transfer where desirable. In the event that ribs31 are not to be used, the porous metal matrix may likewise extend thefull length of the rocket chamber and may have selective areas ofincreased density of greater heat transfer.

The elongated ribs 31 establish flow channels for the coolant but arenot necessary to obtain the advantages of the invention. Accordingly, arocket chamber may be constructed wherein a continuous, annular ring,configurated to form a throat for the rocket chamber is used in place ofthe slugs 30. As will be explained hereinafter, the slugs may be formedby cutting slots in the porous metal ring.

FIG. 4 is a partial cross-sectional view taken along the line 4--4 ofFIG. 3 and shows the ribs 31 disposed between the inner and outer walls22 and 23 and the porous metal matrix slugs 30.

FIG. 5 is a cross-sectional view similar to FIG. 5 showing an embodimentwithout the ribs 31 wherein the slugs are replaced by a continuousannular ring 30 of porous metal.

The rocket chamber 21, as shown in FIG. 3, may be constructed asdescribed hereinafter. Various techniques involving electroforming,sputtering, diffusion bonding and various machining steps have been usedin the past to construct rocket chambers. However, new steps are nowinvolved due to applicants' use of porous metal matrix between the innerand outer walls of a rocket chamber to increase the heat transfereffectiveness. In one method of making a rocket chamber according to theinstant invention, the inner wall 22 of the rocket chamber isconstructed on a suitably shaped split mandrel by either electroformingtechniques or by spinning; that is, by forcing a metal sleeve orcylinder to conform to the mandrel while the mandrel and the sleeve arerotating. A porous metal matrix in the form of a ring, as shown at 300in FIG. 5 is next formed around the rocket chamber inner wall 22 afterthe outer surface of the wall 22 is machined to the desired dimensions.The porous metal matrix may be formed by wrapping wire around the wall22, by disposing the two halves of a split porous metal matrix ringaround the chamber wall 22 or by positioning slugs 30 of porous metalaround the wall 22, as shown in FIGS. 3 and 4.

If no ribs 31 are to be utilized, either a layer of wax or a layer ofleachable material, depending on whether the outer wall is to be formedby electroforming or sputtering, is formed on the exterior of the rocketchamber wall 22 but excluding the porous metal ring 300. The outerrocket chamber wall 23 is formed by electroforming or sputtering and, ofcourse, will bond to the porous metal matrix ring 300 or slugs 30 if thelatter are used.

If ribs 31 are desired to form coolant channels, grooves must bemachined in the wax layer or leachable layer prior to forming the outerwall 23. This groove-machining operation may also be extended to theporous metal matrix ring 300 whereby the ribs 31 will also be depositedbetween the slugs 30 thus formed and will be bonded to those slugs.

After the outer wall is formed, the split mandrel is removed and theinner surface of the inner wall 22 may be machined to a desired contour.At this point, the porous metal matrix ring 300 or slugs 30 are notbonded to the inner wall 22. To accomplish such bonding, the rocketchamber assembly may be heated to a temperature sufficiently high toproduce bonding between the porous metal matrix ring 300 or slugs 30.

To reduce the temperature of the diffusion bonding step so thatundesirable annealing of the inner wall 22 may be avoided, a relativelylow temperature solder material of the types which are well known in theart may be applied to the outer surface of the wall 22 in the areaswhere the porous metal matrix is to be positioned. The solder materialwill bond the porous metal matrix material to the inner wall 22 withoutresorting to the relatively high diffusion bonding temperatures normallyrequired.

The diffusion bonding step may be advantageously eliminated by machiningaway the inner wall 22 completely after the mandrel is removed. Theinner wall 22 is then reformed, either by electroforming or sputteringtechniques and is applied directly to the surface of the porous metalmatrix ring 300 or the slugs 30. The wax or leachable material is thenremoved from between the inner wall 22 and the outer wall 23, as well asfrom all other areas of the rocket chamber.

The inlet manifold at the downstream end of the rocket chamber and thecoolant exit manifold at the upstream end of the rocket chamber areadded by any of the well-known techniques of the prior art.

The construction of a rocket chamber in accordance with the inventionneed not be restricted to one technique such as electroforming but mayinclude other methods. For example, it may be desirable to form theinner wall of any alloy which may be advantageously sputtered, thisbeing done after the outer wall has been electroformed. On the otherhand, there are instances where it would be preferable tosputter-deposit the outer wall.

It will be understood that those skilled in the art may make changes andmodifications to the above-described invention without departing fromthe spirit and scope thereof, as set forth in the claims appendedhereto.

What is claimed is:
 1. A method of making a rocket chamber comprisingthe steps of:forming a first wall having the configuration of aprescribed inner wall for a rocket chamber; disposing a porous metalmatrix around said first wall; applying a removable material to theoutside of said first wall; depositing a layer of metal over saidremovable material and over said porous metal matrix, said layer ofmetal forming a second wall which is bonded to said porous metal matrix;removing said removable material; and heating said rocket chamber to atemperature sufficient to cause bonding of said porous metal matrix,said first wall and any other metal interfaces.
 2. The method of claim 1wherein said porous metal matrix is formed by wrapping wire around saidfirst wall.
 3. The method of claim 1 wherein said porous metal matrix isformed by positioning a pre-formed, axially split, annular body ofporous metal on said inner wall.
 4. The method of claim 1 and includingthe step of machining longitudinal slots in said removable material andin said porous metal matrix to provide slots in which longitudinal ribsare formed when said second wall is deposited.
 5. The method of claim 1and including the step of coating selected areas of the outside of saidfirst wall with solder prior to disposing the porous metal matrix aroundsaid first wall.
 6. The method of claim 1 wherein after said second wallis formed, said first wall is completely removed and a new first wall isdeposited, said new wall bonding to said porous metal matrix therebyeliminating the heating step of claim
 1. 7. The method of claim 6wherein said porous metal matrix is formed by positioning a pre-formed,axially split, annular body of porous metal on said inner wall.
 8. Themethod of claim 6 and including the step of machining longitudinal slotsin said removable material and in said porous metal matrix to provideslots in which longitudinal ribs are formed when said second wall isformed.